FIGS. 9-11 of the accompanying drawings illustrate a prior art device of the solenoid induction type. The device generally comprises a transmitter F and receiver J. In order to locate a buried power cable R or the like, initially it is necessary to connect the transmitter F and conductor Ra of the cable R as shown in FIG. 9, and then to supply a location current Is having a frequency between 10 Hz-500 kHz to the conductor Ra from the transmitter F.
The location current Is flows into the ground M through the distributed capacitance formed by the conductor Rb, insulator R, and ground M, and then it returns to the transmitter F through the earth electrodes O. Flow of the location current Is into the conductor Ra forms a magnetic field H as shown in FIG. 10 around the power cable R.
A detection current is created in the receiving coil Ja of the receiver J by placing it in the magnetic field H and then interlinking the receiving coil Ja and the magnetic field H. The detection current induced in the receiving coil Ja indicates the depth L1 of the power cable R, and the direction thereof. These quantities are based on the parameters of the detection current and they are indicated to the operator.
To locate a pipe S or the like, a compact originating coil U is inserted into the pipe S as shown in FIG. 11, and a location current Is is provided to this originating coil U from a transmitter F. Flow of the location current Is into said originating coil U generates a magnetic field H around the originating coil U.
Placement of a receiver J equipped with a receiving coil Ja in said magnetic field H causes the receiving coil Ja to generate a detection current. The depth 4 of the pipe S and the direction thereof are calculated on the basis of the parameters of this detection current and they are indicated to the operator.
If there is no other noise source near the power cable R and pipe S, the conventional solenoid type induction type buried object locating device can perform accurate location of the buried object by using a comparatively small location current Is, thus resulting in excellent efficiency. However, the case where there is no noise source is very rare and in general, a noise source exists and this results in a signal produced by both the location current Is and a noise signal and it becomes difficult to make an accurate determination of the depth and horizontal location of the buried object. This is particularly so in the case where the depth L, of the power cable R or pipe S, is large or in the case where the power cable R or pipe S is shielded by some other conductive object.
One possible solution to the noise problem is to increase the magnitude of the location current Is fed from the transmitter Fa and thereby diminish the noise to signal ratio.
However, if the frequency of the location current Is becomes 10 kHz or greater, in many cases the magnitude of the location current Is may be limited by law, and it becomes impossible to increase the location current (Is) and thereby diminish the noise to signal ratio.
Accordingly, it is a primary object of the present invention to solve the aforementioned problems for the conventional solenoid induction type buried object locating device. More particularly, these problems are that if the noise to signal ratio increases by the influence of the external noise and the depth of the buried object is great, it is impossible to perform accurate location. Furthermore, if the frequency of the location current Is becomes 10 kHz or greater, in many cases the magnitude of the location current Is may be limited by law and it is impossible to increase the location current Is and diminish the noise to signal ratio. Thus, the invention provides apparatus for locating a buried object wherein the location capability of the apparatus can be greatly increased by an automatic, and in many cases virtually complete, removal of the noise content from the current detected by a receiving coil Ja of a receiver J.